An impact of non-native species invasions on the Caspian Sea biota.

The Caspian Sea is a large inland brackish basin, vulnerable to invaders due to its long isolation and considerable endemism among its native biota. A brief description of Caspian biota evolution until its modern state is given. The pathways and vectors of invasion and the ways of establishment of non-native species since the early 20th century are summarized. The newly established species are euryphilic, with high ecological plasticity, able to adapt to new environments and to affect their biodiversity. This review is based on unpublished field data, collected in 1999-2019 in the Northern, Middle and Southern Caspian, and on relevant published information. The arrival of non-native species occurred in three periods: (1) in the 1930s, deliberate introductions aimed at enriching commercial stocks and edible resources, (2) since 1952, the construction of the Volga-Don Canal led to the arrival of benthic foulers and macrophytes from ships; (3) since the early 1980s to present, ballast water tanks were mounted on ships, favoring the arrival of phyto- and zooplankton species. Most established non-native species reached the Caspian Sea via the Black Sea. They include both Black Sea native species and non-native species from the North Atlantic areas, which first arrived and established in the Black Sea. Few established non-native species came from brackish water; fresh water fishes were deliberately introduced to develop aquaculture. Though not numerous, these species became dominant in both benthos and plankton communities, where they replaced native Caspian species. Among them, the invading ctenophore Mnemiopsis leidyi, which had no predators, continues to thrive in the Caspian ecosystem, impoverishing its biodiversity and bio-resources. However, lately its natural predator, the ctenophore Beroe ovata, arrived and established in the Southern and Middle Caspian providing a chance for ecosystem recovery, as has already happened in the Black Sea.

Adaptive strategies of Mnemiopsis leidyi A. Agassiz 1865 in different environments of the Eurasian seas.

Comprehensive synthesis of the harmful invader ctenophore Mnemiopsis leidyi adaptive strategies and its validation as of a single polymorphic species has been presented. Its high morphological and physiological variability in different environments were demonstrated. M. leidyi being native for eastern coasts of Americas, since the early 1980s began to invade in the Eurasian seas and now it is recorded in a wide range of recipient habitats. Analysis of M. leidyi morphological and eco-physiological variability, phenology and rate of reproduction was performed for different environments based on author's data and published sources. Prominent morphological features of M. leidyi, previously used to subdivide it in three species, in fact are a phenotypical variability, associated with environmental conditions. In recipient environments, M. leidyi pre-adapts for rapid colonization, due to a high metabolism and reproduction rates. It created extensive populations with the various patterns of annual cycle and distribution and heavily impacted the ecosystems.

Molecular Insights Into the Ctenophore Genus Beroe in Europe: New Species, Spreading Invaders.

The genus Beroe Browne, 1756 (Ctenophora, Beroidae) occurs worldwide, with 25 currently-described species. Because the genus is poorly studied, the definitive number of species is uncertain. Recently, a possible new Beroe species was suggested based on internal transcribed spacer 1 (ITS1) sequences from samples collected in Svalbard, Norway. Another species, Beroe ovata, was introduced to Europe from North America, initially in the Black Sea and subsequently (and possibly secondarily) into the Mediterranean and Baltic Seas. In areas where ctenophores have been introduced, they have often had significant detrimental ecological effects. The potential for other cryptic and/or undescribed Beroe species and history of spread of some species in the genus give reason for additional study. When alive, morphological hallmarks may be challenging to spot and photograph owing to the animals' transparency and near-constant motion. We sampled and analyzed 109 putative Beroe specimens from Europe, using morphological and molecular approaches. DNA analyses were conducted using cytochrome oxidase 1 and internal transcribed spacer sequences and, together with published sequences from GenBank, phylogenetic relationships of the genus were explored. Our study suggests the presence of at least 5 genetic lineages of Beroe in Europe, of which 3 could be assigned to known species: Beroe gracilis Künne 1939; Beroe cucumis Fabricius, 1780; and Beroe ovata sensu Mayer, 1912. The other 2 lineages (here provisionally named Beroe "norvegica" and Beroe "anatoliensis") did not clearly coincide with any known species and might therefore reflect new species, but confirmation of this requires further study.

Invasion pathway of the Ctenophore Mnemiopsis leidyi in the Mediterranean Sea.

Gelatinous zooplankton outbreaks have increased globally owing to a number of human-mediated factors, including food web alterations and species introductions. The invasive ctenophore Mnemiopsis leidyi entered the Black Sea in the early 1980s. The invasion was followed by the Azov, Caspian, Baltic and North Seas, and, most recently, the Mediterranean Sea. Previous studies identified two distinct invasion pathways of M. leidyi from its native range in the western Atlantic Ocean to Eurasia. However, the source of newly established populations in the Mediterranean Sea remains unclear. Here we build upon our previous study and investigate sequence variation in both mitochondrial (Cytochrome c Oxidase subunit I) and nuclear (Internal Transcribed Spacer) markers in M. leidyi, encompassing five native and 11 introduced populations, including four from the Mediterranean Sea. Extant genetic diversity in Mediterranean populations (n = 8, N a = 10) preclude the occurrence of a severe genetic bottleneck or founder effects in the initial colonizing population. Our mitochondrial and nuclear marker surveys revealed two possible pathways of introduction into Mediterranean Sea. In total, 17 haplotypes and 18 alleles were recovered from all surveyed populations. Haplotype and allelic diversity of Mediterranean populations were comparable to populations from which they were likely drawn. The distribution of genetic diversity and pattern of genetic differentiation suggest initial colonization of the Mediterranean from the Black-Azov Seas (pairwise F ST = 0.001-0.028). However, some haplotypes and alleles from the Mediterranean Sea were not detected from the well-sampled Black Sea, although they were found in Gulf of Mexico populations that were also genetically similar to those in the Mediterranean Sea (pairwise F ST = 0.010-0.032), raising the possibility of multiple invasion sources. Multiple introductions from a combination of Black Sea and native region sources could be facilitated by intense local and transcontinental shipping activity, respectively.